Exploring the Electrical Tunability of Tubulin and Microtubules  Aarat Kalra , Jack Tuszynski (University of Alberta, Edmonton, Alberta Canada)   C8

Microtubules are slender cylindrical polymers of the protein a,b- tubulin that play a multitude of intracellular roles. Microtubules orchestrate chromosomal segregation, interact with each other via microtubule associated proteins (MAPs) to form a network for the transport of macromolecular cargo, and regulate cell shape and rigidity. Such intracellular roles rely on the lateral rigidity and longitudinal stiffness of microtubules which allows them to exert large mechanical forces on each other as well as the cell membrane. Interestingly, due to the large negative charge and dipole moment of tubulin dimer, microtubules themselves are highly negative and polar structures. Because of such interesting electrical and mechanical properties, microtubules have been shown to hold promise as nanoscale transporter devices. The device-based potential of microtubules has recently been heightened by discovery that the mechanical stiffness of microtubules is tunable (Isozaki et al., 2015; 2017). Microtubules with varying stiffness values were successfully utilized for the fabrication of a sorting device in the presence of externally applied electric field. In this work, we use varying solvent conditions to induce changes in the electrostatic properties of tubulin. We started out by using light scattering to determine electrophoretic mobility (and hence surface charge) of tubulin, and showed that under specific conditions, tubulin could be induced to acquire a positive surface charge. We subsequently use fluorescence microscopy to show that positively surface charged tubulin is polymerization competent, capable of forming positively charged two dimensional polymers (sheets), as opposed to forming cylindrical structures (microtubules). Our work was finally validated using a fluorescence-based electrophoresis assay. These results indicate that the surface charge on the tubulin dimer itself is reversibly tunable. This property indicates strong potential as a tunable electrical element and chemical detector in nanoscale devices. Charge tunability allows tubulin to influence local ionic flows, and as a consequence, regulate the bioelectrical properties within a neuron. This may provide an important insight into the electrodynamic interactions between the action potential and the axonal cytoskeleton.